The present invention relates to a method of fixing, and in particular of assembling and encasing, a fiber-optic component inside a device by means of a polymer composition. The present invention further relates to an optical device comprising at least one fiber-optic component arranged inside said device and in particular a polymer composition capable of holding in a predetermined position and protecting said optical components inside said device. In addition the present invention relates to said polymer composition.
For the purposes of the present invention, fiber-optic component means one or more optical fibers connected optically in some way, possessing characteristics (for example dimensions, constituent materials or dopants, types of covering, mutual position of the fibers, values of the refractive index of the core and of the outermost layers, etc.) chosen in such a way as to transmit an input optical beam into at least one output optical beam in accordance with a predetermined transfer function.
Examples of fiber-optic components are: fiber Bragg gratings (fibre gratings), active fibers used for the amplification of optical signals, fiber couplers, optical fibers in general (for example single-mode fibers, multimode fibers, polarization-maintaining fibers, dispersion-compensating fibers, dispersion-shifted fibres, fibers used in optical sensors, etc.) as well as components obtained by combining them.
Fibre gratings (xe2x80x9cgratingsxe2x80x9d for short) are generally optical fibers that have, in one portion, a refractive index of the core n and/or of the cladding nc permanently modulated along the fiber""s propagation axis. Gratings reflect, according to various transfer functions, optical signals that have different wavelengths.
When the refractive index of the core n assumes a periodic variation (for example sinusoidal) with amplitude and pitch xcex9 constant along the fiber""s propagation axis, the grating is called uniform.
Apodized gratings have an amplitude of the refractive index of the core n that varies along the fiber""s propagation axis (for example according to a Gaussian profile), whereas chirped gratings have a pitch xcex9 that varies along the fibre""s propagation axis.
In an article xe2x80x9cFiber Grating Spectraxe2x80x9d, Journal of Light Technology, Vol. 15, No. 8, p. 1277-1294, August 1997, T. Erdogan describes various types of fiber gratings and presents theoretical principles for their design and their possible uses in the field of optical telecommunications. The types of gratings considered by the author include, among others, the aforementioned uniform gratings, apodized gratings and chirped gratings.
Fibre components and in general optical devices that comprise fiber components, for example devices for chromatic dispersion, are normally housed in units that protect the component and/or the device and limit their overall dimensions, permitting them to be transported.
Devices for compensating chromatic dispersion, for example of the type comprising an optical circulator and a DCG, are housed in suitable modules such as those produced by the applicant and designated by the abbreviation CDCM (Chromatic Dispersion Compensation Module), for example models CDC 0480, and CDC 016160.
For example, U.S. Pat. No. 5,887,107 describes an optical device consisting of a container, and an optical fiber containing, in one portion thereof, a Bragg grating. The container is moreover provided with a locking element, which constrains a portion of the fiber, and a mandrel around which another portion of the fiber is wound.
An organizer rack for the housing of fiber-optic components, electrical, electro-optical and optical components variously connected, is illustrated in U.S. Pat. No. 5,915,061 in the name of the same applicant. This document describes an optoelectronic apparatus that comprises a casing, inside which are arranged an electronic unit and an optical unit, connected electrically to each other; the optical unit comprises an element on which at least one component is housed, which may be of the optical type, with optical connection or of the electro-optical type. The use of filling materials or adhesives inside known optical devices is also known.
For example, U.S. Pat. No. 5,727,105 describes a device comprising a main container and two side containers, with an optical fiber that is introduced from the side container to the main container. The optical fiber is locked in the side container by means of a silicone resin or an epoxy adhesive.
Moreover, U.S. Pat. No. 5,960,143, which relates to a protective casing of an optical component, describes the use of an adhesive product for fixing an optical fiber to a waveguide and for mechanically fixing an optical fiber to a substrate. This patent also describes the use of a water-repellent lubricant, for example of the so-called mechanical type or silicone-based, for separating the optical component from the container walls.
The preparation of room temperature vulcanizable (RTV) and high-temperature vulcanizable (HTV) silicone elastomers (or rubbers) is described in the reference book xe2x80x9cSiliconesxe2x80x94Chemistry and Technologyxe2x80x9d s.v., published by Vulkan-Verlag Essen (DE), 1991, p. 45-59. The RTV silicone rubbers are divided into single-component silicone rubbers (RTV-1) and two-component silicone rubbers (RTV-2). The latter, as stated in the book cited above, can be produced by a condensation reaction between two silicone compounds (for example between a polymethyldisiloxane with xe2x80x94OH end groups and a tetra-ester of silicic acid) or by an addition reaction between two silicone compounds (for example by a reaction of hydrosilation of a silicone compound containing xe2x89xa1SiH groups along the chain with a polydimethylsiloxane containing vinyl groups, either terminal or pendent along the chain).
A first aspect of the present invention relates to a method of fixing an optical component arranged inside a housing, comprising the steps of:
arranging said optical component inside said housing;
embedding said optical component in a polymer composition based on polysiloxanes;
crosslinking said polymer composition, to obtain a crosslinked silicone elastomer capable of fixing said component;
wherein said composition can be crosslinked by an addition reaction and that said silicone elastomer evolves a quantity of hydrogen less than about 1 cm3/kg of elastomer, when submitted to thermal ageing for 15 days at 100xc2x0 C.
Another aspect of the present invention relates to an optical device comprising
at least one fiber-optic component;
a housing capable of containing said fiber-optic component; and
a polymer composition capable of holding said optical component in a predetermined position inside said housing and of protecting said component from mechanical stresses;
wherein said polymer composition comprises a silicone rubber crosslinked by an addition reaction, said rubber evolving a quantity of hydrogen of less than about 1 cm3/kg of rubber as a result of thermal ageing for 15 days at 100xc2x0 C. Preferably, the quantity of hydrogen evolved is less than about 0.1 cm3/kg. Yet another aspect of the invention describes the polymer composition used in practising the invention.